In most commercial printing processes (for example flexography, intaglio printing and lithography), printing ink is transferred to a paper surface through direct mechanical contact. Since the printed image is formed by a number of small screen dots having diameters of 5-50 μm, it is important to achieve good contact over the entire printing surface. If at all places on the printing area of the paper such a contact is not obtained, ink dots not coming into contact with the paper surface will be missing on the printed surface (“missing dots”) which causes blank surface portions or, sometimes in case of multi-colour printing, tinting.
A number of different solutions are known in order to investigate properties of different paper surfaces with regard to the “topography” of the paper in order to assign a value for the paper's ability to correctly receive prints without the occurrence of “missing dots”. With the topography of a material surface is here intended the roughness which is caused by small differences in height in the material surface.
The most common methods for measuring the roughness of a paper can be grouped in the following manner:
Test printing in a commercial printing press or a printer in laboratory outfit, where a printing ink is used in order to evaluate the number of missing printing points.
Profilometry, where a stylus is drawn in a straight line over the surface and the topography is recorded.
Air leakage, where a measuring ring is pressed against the test surface and the air leakage between the inside and the outside of the ring is measured.
Optical non-contacting measurement methods, using for example a confocal microscope or can be calculated from a shadow of light which incides at the surface at a small angle.
“The Chapman method”, wherein a paper with the aid of a pressuring body is pressed against a flat glass prism (Chapman prism).
During a determination according to the last mentioned method, the paper will lie with larger or smaller portions pressed against the glass of the prism. At points where the contact surface of the prism lacks contact with the test surface, obliquely collimated light inciding against the contact surface will be reflected inside the glass body of the prism and these points thereby appear as dark surfaces in the picture registered by the camera.
In the portions of the contact surface against which the paper is in contact, the light will refract through the contacting surface of the prism and illuminate underlying portions of the paper. These illuminated portions will thereby be light. A picture taken through the prism perpendicular to the clamping surface, will therefore have light and dark portions. This picture is subsequently evaluated to determine the topography of the paper according to the above.
All established measuring methods have one or more drawbacks, since on the one hand they may be time-consuming to use, on the other hand they give insufficient detail information about the properties of the contact surface. Some measuring methods use printing inks which affect the measuring result. Most of them lack pressurizing load during measurement.